Over the years a number of theories relating odorant
quality to molecular structure have been proposed. Here we
will review the two most prominent theories and add another
involving the direct participation of certain
neurotransmitters or their hydrolysates in assisting the
docking of odorant molecules with the olfactory receptor
protein.

The Steric Theory of Odor

In 1946, future Nobel laureate, Linus Pauling26
indicated that a specific odor quality is due to the
molecular shape and size of the chemical. Similarly, in the
book, "Molecular Basis of Odor" by John Amoore27,
he extended the idea of a "Steric Theory of Odor" originally
proposed by R.W. Moncrieff in 194928 that stated
air borne chemical molecules are smelled when they fit into
certain complimentary receptor sites on the olfactory
nervous system. This "lock and key" approach was an
extension from enzyme kinetics. Amoore proposed primary
odors (ethereal, camphoraceous, musky, floral, minty,
pungent and putrid). The molecular volume and shape
similarity of various odor chemicals were compared (by
making hand prepared molecular models and physically
measuring volume and creating silhouette patterns - there
were no computer molecular modeling programs in that
era).

The steric theory is well suited to the idea that the
odorant receptor proteins accept only certain odorants at a
specific receptor sites. The receptor is then activated ( by
conformation deformation?) and couples to the G-protein and
the signal transduction cascade begins.

The Vibrational Theory of Odor

In 1938, Dyson29 suggested that the infrared
resonance (IR) which is a measurement of a molecules
vibration might be associated with odor. This idea was
popularized by R.H. Wright in the mid 1950s as
infrared spectrophotometers became generally available for
such spectral measurements which Wright was able to
correlate with certain odorants.30

During the 60s and early 70s, vigorous debate
raged as to the validity of each theory for classifying
chemical odorants.

By the mid-70s, it appeared that Wrights
theory had failed a critical test. The optical enantiomers
of Menthol31 and of Carvone32 smelled
distinctly different, although the corresponding infrared
spectra were identical. And this theory fell from favor.
Recently (August 2001), Leffingwell
has published on the internet an extensive site that
provides over 100 enantiomeric pairs of odorants that have
differing odor properties. This site provides both 2-D and
3-D molecular structures along with odor descriptors, odor
thresholds and original references.

Vibrational Induced Electron Tunneling Spectroscope
Theory

Until the seminal dissertation of Luca Turin33
in 1996, the vibrational theory had been placed under a very
dark cloud. Turin, however, has attempted to provide a
detailed and plausible mechanism for the biological
transduction of molecular vibrations that, while not
accepting the mechanical vibrational spectroscopy theory
previously proposed, replaces it with a theory that the
receptor proteins act as a "biological spectroscope". What
was proposed is a process called "inelastic electron
tunneling". Since this paper, which appeared in Chemical
Senses in 1996 is available for downloading off the Internet
[at http://chemse.oxfordjournals.org/content/21/6/773.full.pdf
], I will only outline the process of electron transfer
proposed.

Suffice it to say that the receptor is triggered by an
odorant in the presence of NADPH (ß-Nicotinamide
Adenine Dinucleotide Phosphate, Reduced Form), which is
formed by the enzymatic reduction of ß-Nicotinamide
Adenine Dinucleotide Phosphate (NADP). NADPH is widely
distributed in living matter and acts as an enzyme cofactor.
ß-NADPH is a product of the pentose phosphate pathway,
a multifunctional pathway whose primary purpose is to
generate reducing power in the form of ß-NADPH.
ß-NADPH transfers H+ and 2e- to oxidized precursors in
the reduction reactions of biosynthesis. Thus, ß-NADPH
cycles between catabolic and biosynthetic reactions and
serves as the carrier of reducing power in the same way that
ATP serves as the carrier of energy.34

NADPH (as Sodium salt)

Since according to Turins theory the receptor
functions as an "NADPH diaphorase", it may be significant
that high levels of diaphorase activity have been detected
in olfactory receptor neurons.35

In order for such electron transfer to occur, Turin
proposes from molecular modeling that a zinc binding site is
present both on the odorant receptor protein and the
G-protein. Zincs involvement in olfaction, its ability
to form bridges between proteins, its presence in electron
transfer enzymes, such as alcohol dehydrogenase and the
presence of a the redox-active amino acid cysteine in the
receptors zinc-binding motif all point to a possible
link between electron flow and G-protein transduction.
"Suppose the zinc-binding motif on the olfactory receptor is
involved in docking to the olfactory G-protein g(olf), and
that the docking involves formation of a disulfide bridge
between receptor and G-protein. One would expect to find on
g(olf) the other half of a zinc coordination site, for
example two histidines in close proximity, and a cysteine
nearby." A search in the primary sequence of g(olf) finds
the motif (His-Tyr-Cys-Tyr-Pro-His). This motif has the
requisite properties for docking. It is exposed on the
surface of the G-protein and is known to interact with
G-protein coupled receptors. In the closely-related
adrenergic receptors, a role for cyclic reduction and
oxidation of disulfide bridges has been suggested (Kuhl,
1985)36 . It involves cross-linking of the
G-protein to the receptor by an S-S bridge which is then
reduced upon binding of the (redox-active) catecholamine to
the receptor, thereby releasing the G-protein. Turin
proposes that a similar mechanism may be at work in
olfaction.

Electron tunneling basically is the transfer of electrons
down the backbone of the protein and here this would only
occur as follows:

"When the (olfactory) receptor binding site is empty,
electrons are unable to tunnel across the binding site
because no empty levels are available at the appropriate
energy. The disulfide bridge between the receptor and its
associated G-protein remains in the oxidized state. When an
odorant (here represented as an elastic dipole) occupies the
binding site, electrons can lose energy during tunneling by
exciting its vibrational mode. This only happens if the
energy of the vibrational mode equals the energy gap between
the filled and empty levels. Electrons then flow through the
protein and reduce the disulfide bridge via a zinc ion, thus
releasing the G-protein for further transduction steps.

If there is a molecule between the electron source and
electron sink, and if that molecule vibrates then (taking
the energy of the vibrational quantum as E) indirect
tunneling can occur by an additional channel if there is an
energy level in the source with energy E above that in the
sink. After tunneling, the molecule will have a vibrational
energy higher by E. In other words, tunneling occurs only
when a molecular vibrational energy E matches the energy
difference between the energy level of the donor and the
energy level of the acceptor. The receptor then operates as
a spectrometer which allows it to detect a single
well-defined energy, E . If the change in energy between
donor and acceptor levels is sufficiently large, tunneling
current flows across the device only when a molecule with
the appropriate vibrational energy is present in the gap. If
there are several vibrational modes, which one(s) get
excited will depend on the relative strengths of the
coupling, and that may be expected to depend, among other
things on the partial charges on the atoms and the relative
orientation of the charge movements with respect to the
electron tunneling path."33

While Turins theory has not been validated, it
seems quite plausible. However, even if generally valid, it
does not necessarily mean that the "Steric Theory"
doesnt play a role.

[The so-called electron tunneling concept in proteins
is a major topic of debate as to the exact mechanism of
electron transfer. This stems from the work of Jacqueline
Barton (Electron transfer between metal complexes bound to
DNA: is DNA a wire?) at the California Institute of
Technology.37 ]37a

While both the "Steric" and "Vibrational Induced Electron
Tunneling Spectroscope" theories answer many of the
questions posed, as one is solved, others arise.

Ribonucleotides as the Odorant carrier?

It is now obvious, perhaps, that a multiplicity of events
occur in olfaction. But several major questions that have
not been addressed remain to be answered.

1. Are certain neurotransmitters (or their
hydrolysates) involved not just as so-called "second
messengers" in the transduction cascade, but are they
also involved as "Amplifiers" that help to capture the
odorant molecules and direct them to the receptor sites?

2. Are the ribonucleotides (AMP, cAMP, GMP and cGMP),
[as well as possibly IP3], the glue that helps to
bind odorants into the odorant receptor sites?

While there are a few intriguing clues in the literature
relative to these questions, it appears that the potentially
powerful electrostatic affinity properties of some of these
neurotransmitters (or their hydrolysates) may possibly play
a significant role early in the olfactory process.

In the chemoreception of "taste", it has long been known
that certain ribonucleotides (especially 5-guanosine
monophosphate [5-GMP] and 5-inosine
monophosphate [5-IMP]) have potent synergistic
effects with MSG (monosodium glutamate)38
including a significant lowering of the MSG threshold level.
In 1980, Torii and Kagan showed that a several-fold
enhancement of binding of glutamate occurred with bovine
taste papillae in the presence of certain 5'-ribonucleotides
(e.g., 5'-GMP, 5'-IMP) but not with others (e.g.,
5AMP).39

Now it should be noted that 5-IMP and 5-GMP
(as their sodium salts) commercially are used extensively as
flavor enhancers, especially for meat and fish products to
enhance meaty, brothy and the "uamani" character (often in
conjunction with MSG to take advantage of the synergistic
flavor enhancement).

It has also been observed that there is a large synergism
was observed between MSG and two species of nucleotides (GMP
and IMP) in most mongrel dogs40 and between MSG
and three species of nucleotides (GMP, IMP, and AMP) in
beagles. This has also been observed for GMP and glutamate
in mice.41

Chemically, it should also be noted that 5-inosine
monophosphate is the product of enzymatic deamination of
5-adenosine monophosphate. For example, In meat
extracts, after slaughter, there is as rapid transformation
of 5-ATP to 5-AMP to 5-IMP.

In olfaction, very few studies are available that show
activity of these ribonucleotides in enhancing olfaction.
However, Getchell has demonstrated that 8-Bromo-cAMP applied
to the ciliated side of the mucosa of the bullfrog caused a
concentration-dependent, reversible increase in the basal
short-circuit current, but not when it was applied to the
submucosal side. Pulses of 8-Bromo-cAMP and odorant
presented simultaneously resulted in currents that added
nonlinearly.42

In addition, 5'AMP odorant binding sites on the dendrites
of the olfactory receptor neurons in the sensilla of the
spiny lobster are distributed along the entire dendritic
region that is exposed to odorants. The distribution of
these 5'AMP binding sites is considered much more extensive
than that of enzymes that inactivate 5-AMP.43

The few implications of ribonucleotides to playing an
active part in what I will refer to as the extracellular
side if the receptor neurons in the mucosa has largely been
overlooked probably due to two factors: (1.) the
ribonucleotides are largely water soluble and have not been
examined as possibly complexing with lipid odorants and (2.)
researchers have focused on the intracellular second
messenger activity of such compounds.

Results in our laboratory using molecular modeling and
molecular fitting programs, however, show that 5AMP,
5cAMP, 5GMP, 5cGMP and IP3 all demonstrate
dramatic electrostatic affinity for fitting with many
odorants. For example, when compared to fitting with
5-ATP, 5-ADP, 5-GTP or 5GDP, the
[computed] electrostatic fitting energy is of an
order of magnitude 105 - 106 more
favored. In addition, with certain odorants in a related
series that have similar odor properties, we see similar
fitting patterns for certain conformers [An explanation
of conformers will follow in a subsequent update]. These
observations are intriguing, since, if such electrostatic
forces assist the odorant via some sort of complex in
fitting into the receptor this may be a "first" step in
the transduction process.